This application claims the benefit of Japanese Application Number PCT/JP02/2114 filed Mar. 7, 2002, the entirety of which is incorporated by reference.
1. Field of the Invention
The present invention relates to a ceramic envelope for a high intensity discharge lamp, to be used in a high intensity discharge lamp such as a high-pressure sodium lamp or a metal halide lamp. Further the present invention relates to a method for producing a polycrystalline transparent sintered alumina body adapted for use in such ceramic envelope. More particularly, it relates to a method for producing a polycrystalline transparent sintered alumina body adapted for use in a ceramic envelope for a high intensity discharge lamp, by optimizing kinds and amounts of sintering aids to be added to the alumina.
2. Related Background Art
A polycrystalline transparent sintered alumina body, being excellent in transparency, heat resistance and anticorrosion resistance, is widely employed in a ceramic envelope for a high intensity discharge lamp such as a high-pressure sodium lamp or a metal halide lamp.
Such polycrystalline transparent sintered alumina body is formed with an addition of a sintering aid such as MgO, La2O3 or Y2O3 in order to improve a light transmittance and a mechanical strength. An effective amount of addition of such sintering aid is described for example in Japanese Patent Kokoku Publication (JP-B) Nos. 60-48466 and 61-6023 and in German Patent No. DE 2042379.
Japanese Patent Kokoku Publication (JP-B) No.60-48466 describes additions of MgO by 0.001 to 0.1 wt. %, La2O3 by 0.001 to 0.05 wt. % and Y2O3 by 0.001 to 0.05 wt. % with a ratio MgO/(La2O3+Y2O3) within a range of 0.5 to 2.0, and that such ratio enables to sinter at a low temperature and to provide an optical transmittance and a mechanical strength of an excellent level.
Also Japanese Patent Kokoku Publication (JP-B) No. 61-6023 discloses a configuration of improving an in-line transmittance by maintaining MgO at 150 ppm (0.015 wt. %) or less, and German Patent No DE 2042379 discloses a ratio of MgO within a range of 0.01 to 0.1 wt. %, La2O3 within a range of 0.05 to 0.5 wt. % and Y2O3 within a range of 0.05 to 0.5 wt. %.
However, such prior technologies have the following problems. First, when MgO is added in a large amount, it causes solid dissolution with alumina to form an oxygen lattice defect at the grain boundary. As a result, a blacking phenomenon is observed. Also, MgO has a strong hygroscopic property and moisture absorbed in the envelope is broken down into hydrogen and oxygen. Then the hydrogen induces extinction of an electric discharge arc, while the oxygen accelerates oxidation of electrodes.
On the other hand, when MgO is added in a small amount, sintering aids reduce their effectiveness. For example, crystal grains are not controlled to be out of uniformity, which lowers mechanical strength. Moreover, transparency is reduced due to a failure of complete bubble discharge in the grains.
The present invention is to provide a ceramic envelope for a high intensity discharge lamp, formed by a polycrystalline transparent sintered alumina body obtained by sintering alumina under additions of MgO within a range from 0.002 to 0.010 wt. % and La2O3 within a range from 0.005 to 0.020 wt. %, in such a manner that a sum of MgO and La2O3 is 0.030 wt. % or less and that a ratio thereof satisfies a relation 0.2xe2x89xa6MgO/La2O3xe2x89xa60.5.
The present inventors have experimentally found optimum kinds and amounts of sintering aids adapted for use in the ceramic envelope, formed by alumina, for the high intensity discharge lamp. Addition of MgO and La2O3 is essential for controlling a size of alumina, crystal grains and a wintering temperature, and for extending the service life of the discharge lamp. However, a total amount of addition exceeding 0.030 wt. % results in a blacking of the envelope or a deterioration in the total transmittance. Also a ratio of MgO and La2O3 outside the above range results in a deterioration in the uniformity of grains or in a luminous flux maintenance (blacking of envelope). Therefore, an addition of the sintering aids in the above-mentioned condition allows to obtain a ceramic envelope excellent in uniformity of grains, total transmittance, luminous flux maintenance and durability, which enables to obtain a ceramic envelope suitable for the high intensity discharge lamp.
A method for producing a polycrystalline transparent sintered alumina body of the present invention includes the steps of: adding and mixing, to powdered alumina of a purity of 99.9% or higher, MgO within a range from 0.002 to 0.010 wt. % and La2O3 within a range from 0.005 to 0.020 wt. %, in such a manner that a sum of MgO and La2O3 is 0.030 wt. % or less and that a ratio thereof satisfies a relation 0.2xe2x89xa6MgO/La2O3xe2x89xa60.5, firing a mixture of alumina at 900 to 1200xc2x0 C., molding the pre-sintered alumina, and sintering the molded alumina in a reducing atmosphere under a normal pressure.
Such producing method allows to obtain a polycrystalline transparent sintered alumina body excellent in uniformity of grains and in transparency, by a simple sintering under the normal pressure.